Ultraviolet light (UV) radiation has long been considered a viable method for drinking water disinfection due to its ability to inactivate protozoa and other microbial species. UV of a given wavelength is absorbed by the cellular nucleotides of bacteria, viruses and other microorganisms and causes cross-linking, or demerization, of their RNA and DNA, thereby destroying their ability to multiply and thereby effectively disinfecting the water. Further, UV light radiation does not create significant disinfection by-products. However, due to the cost that is directly proportional to power requirements, UV disinfection can be very expensive to implement. Power requirements for UV disinfection depend primarily on the required fluence, or the product of irradiance and exposure time.
Ion generators have also been employed in previous attempts to control algae, nuisance invertebrates, and microorganisms. Such ion generators are based on well-known principles of electrochemical reactions, one of which is referred to as electrolysis. Electrolysis is an electrochemical process by which electrical energy is used to promote chemical reactions that occur on the surface of functionally cooperating electrodes. One electrode, called the anode, involves the oxidation process where chemical species lose electrons. A second electrode, called the cathode, involves the reduction process where electrons are gained. In water, for example, oxygen is generated at the anode and hydrogen is generated at the cathode. The generation of hydrogen and oxygen in fresh water by the process of electrolysis will be weak due to the low electrical conductivity of the water. The oxygen generated aids in the prevention of the deposit of inorganic salts on the electrodes. The function of an ion generator is also to produce metal ions, typically copper ions or silver ions. Metal ion production is accomplished by use of an electrically charged metal anode that comprises atoms of the metal ions that are to be generated. It is the purpose of the ion generator to feed the metal ions out of the generator before they can be deposited on a cathode. The metal ions and oxygen, both of which are produced by the ion generator, are feed into the water stream of the water system to prevent fouling of the system by algae, nuisance invertebrates, microorganisms, and inorganic salts. As previously mentioned, these inventors have devised ion generators utilizing these principles and which are the subject of U.S. Pat. Nos. 6,350,385; 6,800,207; and 6,852,236 issued to Holt, et al.
The toxicity of copper and silver to aquatic organisms is well established although the exact mechanism is not well defined. The bactericidal effects of silver, for example, have been known for centuries. Silver has been shown to be effective as a disinfectant against coliforms and viruses, including human adenoviruses, as well as other microbial species. In general, these heavy metals must be in an ionic form in order for them to be toxic to invertebrates, microorganisms and algae. The eradication of microorganisms is attributed to positively charged ions that are both surface active and microbiocidal. These ions attach themselves to the negatively charged bacterial cell wall of the microorganism and destroy cell wall permeability. This action, coupled with protein de-naturation, induces cell lysis and eventual death. One advantage to the use of metal ionization, for example, is that eradication efficacy is wholly unaffected by water temperature. Chlorine, a commonly used antifouling chemical, is somewhat temperature dependent. Furthermore, the metal ions actually kill the microorganisms, and other microorganism-promoting bacteria and protozoa, rather than merely suppress them, as in the case of chlorine. This minimizes the possibility of later re-colonization. Other advantages of metal ionization compared to other eradication techniques include relatively low cost, straightforward installation, easy maintenance, and the presence of residual disinfectant throughout the system. In water, and at concentrations sufficient for bactericidal activity, silver does not impart taste, color or odor and has no apparent detrimental effects on mammalian cells. Accordingly, the United States Environmental Protection Agency (USEPA) does not set a primary drinking water standard for silver.
The photochemistry of silver salts, or silver compounds, is also well known. When silver salts are exposed to light, silver ions and free electrons are generated which, in turn, combine to form silver atoms. The silver atoms produce the “latent image” which is enhanced through the development process.
In the view of these inventors, what is needed is an ion generating and UV generating disinfection system that uniquely combines silver photochemistry principles, heavy metal toxicity, and UV light radiation to form a highly effective combined water disinfection method and system. Such a combination would be highly lethal to a broad range of microbial organisms, including viruses, because it would synergistically improve the disinfection or bactericidal effects of ion generation or UV radiation working individually. This synergism occurs because, for example, silver ions complex with the DNA of microorganisms, making them even more susceptible is and less resistant to the bactericidal effects of UV radiation. Such a combined method and system would, in effect, work to immediately kill most of the microorganisms and then cause a residual killing mechanism to greatly enhance the water disinfection process. In the view of these inventors, what is needed is such a method and system whereby the system can be configured for single pass through, dual pass through or for recirculation such that the order of exposure to the ion generation and UV radiation aspects can be varied, altered, or combined as desired or required by any particular application. What is also needed is such a method and system that includes means for controlling ion concentration and UV fluence levels to maximize performance, to minimize energy consumption, and, in some situations, to selectively target certain microorganisms for inactivation.